refactor: decouple UserRepository Google auth contract

[jjoonleo]

Summary

• Remove Google Sign-In SDK types from the domain UserRepository contract.
• Add an app-owned GoogleAuthCredential plus a GoogleAuthenticationService adapter that owns SDK initialization, auth events, cancellation translation, and disconnect.
• Update mobile/default and web Google sign-in paths so provider authentication returns app-owned credentials before UserRepository establishes the OnTime session.
• Clean repository/domain/use-case fakes so they no longer import google_sign_in just to satisfy UserRepository.

Closes #544

Tests

• dart run build_runner build --delete-conflicting-outputs
• flutter analyze
• flutter test test/domain/repositories/user_repository_boundary_test.dart test/data/repositories/user_repository_impl_test.dart test/presentation/login/screens/sign_in_main_screen_test.dart test/domain/use-cases/delete_user_use_case_test.dart test/domain/use-cases/user_use_cases_test.dart test/domain/use-cases/schedule_mutation_use_cases_test.dart test/domain/use-cases/reconcile_alarms_use_case_test.dart test/core/dio/interceptors/token_interceptor_test.dart test/presentation/my_page/delete_user_modal_test.dart
• flutter test